In data processing systems, magnetic disc memory systems are frequently used because they have large storage capacity and require a relatively short time for a magnetic read/write head to access data contained anywhere on discs of the memory from the moment the head receives an instruction to access the data. The magnetic discs are driven at constant rotational velocity by an electric motor.
A magnetic disc carries coded data usually in binary form, on both faces of the disc in concentric circular recording tracks having a width that does not exceed a few hundredths of a millimeter. The tracks are identified by allotting them an address or serial number j (j being a whole number) from 0 to (N-1), where N is the total number of recording tracks. The tracks having addresses (j-1) and (j+1) are adjacent tracks to track j.
Memories having a small storage capacity contain a limited number of discs (normally only one or two). In small memories, data are stored, i.e., recorded, on each of the faces of the discs by setting a large amount of space aside for the data intended to be processed by the data processing system of which the memories are a part. A small amount of space is set aside for data that enables the tracks to be located; these data indicate the track addresses and enable the magnetic head to be servo controlled to a position above the tracks. In the small area are also stored data which indicate whether or not the tracks contain faults.
For the sake of simplicity, a memory is considered which contains only a single disc. Preferably, each face of the disc is associated with a single magnetic read/write head, i.e. a magnetic read/write transducer. Current practice, as described in commonly assigned U.S. Patent application Ser. No. 765,058 filed on Feb. 2, 1977 entitled "Method of Writing Addresses on a Magnetic Recording Medium", now U.S. Pat. No. 4,151,571, issued Apr. 24, 1979, is for the data contained on each face of the disc to be distributed over equal adjacent circular sectors S.sub.0, S.sub.1 . . . S.sub.i . . . S.sub.n. Generally, one face of a disc is divided into several tens of sectors (usually 40 to 50).
When the magnetic disc face associated with the magnetic head passes in front of the head, sectors S.sub.0, S.sub.1, S.sub.2 etcetera are read by the head in sequence. It is therefore said that sector S.sub.0 precedes sector S.sub.1, that sector S.sub.1 precedes sector S.sub.2, that sector S.sub.i precedes sector S.sub.i+1, and so on. In more general terms, if two items of information B.sub.k-1 and B.sub.k which follow one another along the same track j on the face are considered, item B.sub.k-1 precedes item B.sub.k if item B.sub.k-1 is read by the head before B.sub.k, or that item B.sub.k follows item B.sub.k-1.
Each sector S.sub.i is divided into two unequal areas. The larger area contains the data to be processed by the data processing system of which the disc memory is a part, while the smaller area contains data for locating the tracks and indicating faults. For the sake of simplicity, the data contained in the larger area is referred to as "data to be processed". In each sector, the smaller area is divided into a plurality of reference zones, one for each track so each track is associated with a single reference zone.
It is recalled that a bit is a binary 1 or 0 digit. The 1 or 0 may be expressed on a magnetic medium or as an analog or logic electrical signal. A logic signal is capable of assuming only two values called "logic or binary zero" and "logic or binary one"; an analog signal is a signal having a voltage that may vary continuously between two positive and/or negative extreme values. Any item of data or information recorded on the disc is referred to herein as a "bit".
To record a series of data items on a magnetic disc, a succession of small magnetic domains termed "elementary magnets" are formed on each track. These domains are distributed along the entire length of the track and have magnetic inductions with the same modulus and of successively opposing senses in a direction parallel to the surface of the disc. A data bit is represented by a change in the sense of magnetic induction, also termed a magnetization sense change. There are two different types of sense changes, namely:
when the magnetic head passes successive elementary magnets having negative and positive induction on the disc, the magnetization sense change is positive; and
when, on the other hand, the head passes successive elementary magnets having positive and negative induction, the magnetization sense change is negative.
The address of a track contains a number (p) of bits such that 2.sup.p is less than or equal to the number of tracks n. Each reference zone in sector S.sub.i, associated with a track having an address j, contains n cells (n being a whole number) C.sub.1, C.sub.2 . . . C.sub.k . . . C.sub.n. The cells are preferably arranged so each of a number (p) of cells contains two bits; one bit represents a portion of position control information, while the second bit represents a portion of the address for the track of serial number j. Another type of cell contains two fault-indicating bits which indicate whether a portion of track j within sector S.sub.i+1 (following sector S.sub.i) does or does not contain faults. The cells are described in further detail in the commonly assigned U.S. application entitled "Method of Writing Information Relating to Faults in a Magnetic Recording Medium" filed as Ser. No. 835,402 on Sept. 21, 1977, now U.S. Pat. No. 4,152,695, issued May 1, 1979.
The two magnetization sense changes which correspond to the two bits of each cell are of the same kind. Each change can occupy one of only two predetermined positions in the cell. The value of the bit represented by the change depends upon the position which the change occupies, as described in above mentioned U.S. Pat. No. 4,151,571. Thus, if a cell in a reference zone containing track locating data is considered, the position-control bit corresponds to the first change while the track address bit for the zone is the second change. If the magnetization sense change corresponding to the address bit occupies a first position (the position first encountered by the magnetic read head when the face of the magnetic disc which is associated with the head passes in front of it) the bit is equal to 0. If the change in magnetization occupies the other of the two predetermined positions, termed the "second position", the bit is equal to 1. The same rules apply both to the position-control bits and to the fault indicating data.
When the magnetic head encounters a series of magnetization sense changes representing a reference zone, it emits a series of analog signals which are shaped into a series of logic pulses by shaping circuits. The beginning of a reference zone is indicated by a spacial pulse. As described in U.S. Pat. Nos. 4,151,571 and 4,152,695, the boundary between two reference zones of one and the same sector which correspond to two adjoining tracks having serial numbers j and (j+1) coincides with the circular axis of symmetry Ax.sub.j of magnetic of serial number j.
It is assumed that "data to be processed" recorded on a magnetic track of serial number j is read by the magnetic head associated with the face which carries this data only when the head is perfectly centered on the circular axis (Ax.sub.j) of symmetry of track j. Such an assumption enables the reading to take place with maximum accuracy because the magnetic head is centered in any sector S.sub.i, on the boundary between two reference zones corresponding to tracks with addresses of j and (j+1). Thus, the reading from zones j and (j+1) occurs with the head positioned at the same radial position as when the head reads the data to be processed. Thereby, data are read from the two reference zones when the magnetic head is straddling them.
The head cannot move to the radial position in the reference zones from the radial position it occupies while reading the data to be processed because of:
(1) the high speed the disc moves past the head; and
(2) the small length within each sector of the reference zone associated with track j in comparison with the length of the track portion containing the "data to be processed".
In conclusion, within the overall quantities of data items contained in sector S.sub.i, the reference zones, as a whole, contain a sub-set of data items, each of which is defined by a magnetization sense change. The changes are recorded, i.e. stored, in a plurality of adjoining tracks which are read by a magnetic head straddling two adjoining tracks. The head straddling can cause the electrical output signal of the head to have one of two possible waveforms regardless of whether the head senses fault indicating or position controlling bits. The two waveforms can occur because two address bits at the same position in two reference zones which correspond to two adjoining tracks in the same sector S.sub.i, are simultaneously read. Thus the bits of the k.sup.th cell in the two zones, one having address j and the other having track address (j+1) are read at the same time by the head.
The first waveform occurs when the two address bits have the same value, that is when the bits occupy the same predetermined position within their respective cells. In such an instance, the head output signal is a pulse having an amplitude A, which results from the superimposition of two pulses having an amplitude A'=A/2, where A is the amplitude of the pulse signal when intended data are read. The value of the two read bits is then determined in the manner described in U.S. Pat. No. 4,151,571. In particular, the position of the pulse is identified in time relative to a pulse which indicates the beginning of the above mentioned two reference zones.
The second waveform occurs when the two address bits have different values, that is when like numbered bits of zones j and (j+1) occupy two different predetermined positions. In such a case the head derives a waveform including two successive pulses, each of amplitude A' and corresponding to one of the two bits. The time interval (t) between the two pulses is equal to the ratio d/v, where d is the distance separating the two predetermined sense change positions and v is the rotary speed of the disc. In such a case the output signal emitted by the head is an "uncertainty signal" made up of two "uncertain bits". The value of each of the two bits may, for example, be determined in the manner described in the commonly assigned U.S. Pat. application Ser. No. 753,809 entitled "Methods of Shifting a System Which is Movable Relative to a Carrier for Recorded Information and Arrangements for Putting the Method Into Practice" now U.S. Pat. No. 4,166,970, issued Sept. 4, 1979. The prior art method of writing data in the reference zones of a magnetic disc has the following disadvantages:
(1) inaccurate centering of the head relative to the axis of the track and thus relative to the boundary between two adjoining reference zones,
(2) variations in the distance between the head and the face of the disc which is associated with it, and
(3) instantaneous variations in the disc speed cause considerable variation in the amplitude of the signal derived by the head so there is a likelihood that a signal which should be of the first type (having an amplitude A) is transformed into an uncertainty signal of the second type and vice versa. This gives rise to the risk of error in determining the value of the bits written in the reference zones.